This invention relates to video filters, and is more particularly directed to trap filters or blocking filters of the type that are employed in a cable video system. These filters are typically employed either to block a premium or pay channel but to pass other channels, or to block out a narrow band of frequencies from within a particular video channel where single or multiple scrambling signals have been injected into a pay television channel.
A critical requirement of these filters is that they be factory tuned to reject or pass specific signals, and that they have a high degree of stability and reliability in the outdoor operating environment. It is a further requirement that these filters be economical to construct and that precise tuning to the proper frequency be easy to carry out.
Current state-of-the-art video filters commonly have an attenuation of approximately 75-80 dB at the center frequency of their filter notch. However, it has long been desired in the industry to obtain an attenuation of 90 dB or better, so that the pay channel is passed with a minimum of signal degradation. State-of-the-art video trap filters are described, e.g., in U.S. Pat. Nos. 4,701,726 and 4,451,803.
It is also desired that any filter assemblies be compatible both with the conventional on-pole placement, (i.e., off the customers premises), and also with the on-premises enclosure that has become a current trend in the cable industry. In the latter case, the traps, filters, signal splitters and the like would be located on the customer's premises, rather than on a pole, and the video signals could be split to serve more than one set in the customer's home. A typical family can have two or more television sets, and some family members may want to watch different types of programming, such as news and sports, at the same time.
The off-pole, on-premise enclosure permits a new geometry to be employed for the traps, because it is possible to avoid the port-to-port round-shape restriction imposed on the earlier traps. With alternative geometries it is possible for the on-premise-enclosure traps to pass signals that are much sharper.
At the same time, any traps of new design should be simple to construct, tune, and seal, and should not employ an unusually large number of parts.